It is known to provide detachable integrated circuit memory in the size and form of a thin card, much like a credit card, that is connectable to a memory port of a host data processing system. A typical general-purpose host system, such as a desktop computer, incorporates a processor for utilizing data recorded on the memory card in a variety of applications; a typical specialized host system, e.g., an image capture processing system, provides large volumes of digital data for rapid recording on the card. An example of the latter host system is shown in U.S. Pat. No. 5,016,107, wherein an image sensor generates analog image information that is converted into digital signals, transformed, and encoded into a compressed stream of digital signals that are down-loaded to a removable memory card. The memory card disclosed therein includes a commercially-available high speed static random access memory (SRAM).
Memory cards are also available that include a plurality of non-volatile, electronically programmable read only memory (EPROM) devices. Flash EPROM devices, in particular, are desirable because, unlike SRAM, they are inherently non-volatile and less in cost, size, and complexity. Present flash EPROM personal computer memory cards are designed to substitute for floppy or hard disk drives and, for this purpose, are configured to write and read data files with the same usage and timing as such drives. While the writing speed of flash EPROM technology is faster than floppy or hard drives, it is much slower than the write speed of SRAM and DRAM technology. This ordinarily poses little problem for a personal computer, since the card is replacing the floppy or hard drive and, thus, is not normally intended for applications having more demanding data rates. In such cases, data is simply written seriatim into one memory location after another, until a memory device is filled, then on to the next device; data corresponding to a single event, e.g., an image, is series-linked, or concatenated, in the corresponding memory map. When the host system is an electronic camera, however, the large amount of data generated by high resolution imaging requires a correspondingly high write rate for reasonable image capture speeds.
To avoid turning to DRAM or SRAM technology for high data transfer rates, two approaches to improving the write rate of EPROM cards have been suggested. In U.S. Pat. No. 4,803,554, "Electronic Imaging Camera Utilizing EPROM Memory", issued to Pape on Feb. 7, 1989, an electronic camera includes an EPROM module having sixteen EPROM devices arranged in four groups. A set of four discrete latches are used to latch four successive image data bytes as they are generated by a signal processing circuit. The data bytes are then simultaneously transferred from the latches to the four EPROMS of a selected group. This allows loading each EPROM in the selected group in unison, thereby achieving a 4 to 1 reduction in data transfer time. Successive data bytes are then loaded in a similar manner until all of the data for a given image is loaded within the selected group of four EPROMS; then, the next group of four EPROMS is similarly loaded with the next image, and so on. Another approach is illustrated by Ser. No. 844,855, "Flash EPROM Memory Card for Electronic Camera", filed in the name of N. Reyner on Mar. 3, 1992 and assigned to the same assignee as the present application. Predicated on the fact that the latching cycle of a flash EPROM is much shorter than the programming cycle, this approach shows the interleaving of successive data words between all memory banks on a memory card, such that the full capacity of the memory devices are employed. This has the advantage of allowing a write (program) operation to be initiated in a second memory bank on new data before a write (program) operation is completed in a first memory bank on the preceding data, and so on across all the memory devices in the card.
Notwithstanding these advantages as to writing time for high density data files, it remains desirable to serially load data into each flash EPROM memory element, one at a time, instead of interleaving data among plural flash EPROM memory elements. This is because the erase block for a flash EPROM memory card is usually each flash EPROM memory device (or a relatively large subdivision thereof), meaning that all storage locations of the block of a memory device are erased at once. Consequently, the higher write speed obtained by interleaving has the attendant disadvantage that data pertaining to one event, e.g., an image, is spread across many memory devices. To erase that image then requires that each block of every flash EPROM memory element into which the data is interleaved must be erased, i.e., the effective "erase block" has been significantly increased compared with the case of concatenated storage.